1. Technical Field
This invention relates generally to a piezoelectric device, and more specifically to controlling the temperature induced deformation of a piezoelectric device.
2. Background
Piezoelectric devices, such as piezoelectric actuators, generally consist of a piezoelectric material that deforms when an electric field is applied across it. Additional materials may be bonded with the piezoelectric material, such as metallic layers that act as electrodes, insulating materials to prevent current from flowing between particular areas of the device, and adhesives to bond the various layers together.
One common feature that most piezoelectric actuators have is that they are sensitive to temperature, growing or shrinking in particular dimensions as a function of changes in temperature.
FIG. 1 shows one type of piezoelectric device: a piezoelectric bender actuator 10. A typical piezoelectric bender actuator 10 typically consists of an electroactive layer 12, such as some types of ceramic, disposed between two electrodes 14, although other configurations are also possible. The bender actuator 10 may be pre-stressed by ways known to those skilled in the art to have a domed configuration. Examples of such pre-stressed piezoelectric bender actuators 10 may be found in U.S. Pat. Nos. 5,471,721 and 5,632,841.
The bender actuator 10 may be coupled with any of a variety of moveable objects, such as a rod 16 or a mirror (not shown). A return spring 18 may be coupled with the rod 16 to keep the rod 16 in contact with the bender actuator 10.
FIG. 2 shows the piezoelectric bender actuator 10 when an activation signal, e.g., power, such as an operating voltage or current, is applied. When power is applied to the bender actuator 10, such as a voltage being applied across the electrodes 14, an electric field is induced across the electroactive layer 12. The electric field typically causes the domed actuator 10 to displace in a first direction, such as flatten, as shown in FIG. 2. Alternately, the electric field could cause an increase in doming. When the bender actuator 10 flattens, it may move the rod 16. Typically the stroke of the bender actuator 10 will be calculated to move the rod 16 from a first predetermined position to a second predetermined position, or vice versa (FIG. 1 vs. FIG. 2).
One problem with many piezoelectric actuators 10, including bender actuators, is that they are sensitive to temperature. Many piezoelectric bender actuators 10 change their dome height as a function of temperature. Typically as the temperature drops, the piezoelectric bender actuator 10 will increase its dome height. This poses problems in that the operating voltage will not cause the piezoelectric bender actuator to travel through its intended full stroke length.
For example, a particular piezoelectric bender actuator 10 may have 100 microns of stroke from its rest/domed (no voltage applied; FIG. 1) position to its fully or nearly flattened position (operating voltage applied; FIG. 2). However, temperature induced deformation, e.g., due to cold, may cause an additional 100 microns of doming of the bender actuator 10. FIG. 3 shows one example of temperature deformation of the bender actuator 10. Thus, when the operating voltage is applied to the cold actuator 10 of FIG. 3, it flattens from 200 microns of dome height to 100 microns of dome height, and never becomes fully flattened (0 microns of dome height). In this instance, the bender actuator would only stroke back to its original (non-cold) position shown in FIG. 1.
In actuality, most benders never fully flatten, only flattening by xc2xd to ⅓ of its rest/dome height. However, for illustrative purposes, the embodiments of the invention will be described as having a fully flattened actuated position when full voltage is applied.
Further, the amount of temperature induced deformation will vary with magnitude of the temperature, with very cold temperatures typically causing more doming than less cold temperatures. Thus, the stroke of the bender actuator is often dependent on its temperature, which may be undesirable in many applications.
The present invention provides apparatuses and methods for controlling the temperature induced deformation of a piezoelectric device. A piezoelectric device receives an activation signal and displaces in a first direction for a first predetermined distance as a function of the activation signal. The piezoelectric device also displaces in a second direction as a function of a change in temperature of the piezoelectric device. A stop is located a second predetermined distance from the piezoelectric device. The stop prevents the displacement of the piezoelectric device in the second direction beyond a third predetermined distance. A charge redistributing device is coupled with the piezoelectric device. The charge redistributing device redistributes charge on the piezoelectric device due to the temperature change to relieve internal stresses of the piezoelectric device due to temperature induced deformation of the piezoelectric device.